The invention relates generally to a digital subscriber loop (DSL) system which utilizes un-shielded twisted pair (UTP) or shielded twisted pair (STP) for network connection to customer premises equipment (CPE) through a network interface device (NID) and standard (i.e., in-home) wiring using coaxial cable for CATV and UTP for Plain Old Telephone Service (POTS). The invention also relates to a DSL system which uses a passive NID-based end station and an active CPE-based end station for spectrally relocating xDSL frequency signals to lower noise locations on customer premises wiring.
Digital subscriber loop signaling (e.g., ADSL, HDSL, VDSL and so on which are hereinafter generally referred to as xDSL) provides a method for high speed data transfer across existing telephone lines. Plain Old Telephone Service (POTS) transmission occurs in a frequency range of approximately 0 Hz to 4 kHz. xDSL utilizes a higher set of frequencies from 20 kHz to 1.1 MHz. Using a different frequency band gives xDSL several advantages over current analog modem technology. For example, fast data transmission downstream from the network to the user is achieved (e.g., on the order of 8 Mb/s), as well as improved data transmission speed upstream from the user to the network (e.g., on the order of 640 kb/s). In addition, xDSL allows simultaneous data transfer in both directions (i.e., upstream and downstream) and does not interfere with telephone transmissions. Thus, both telephone and xDSL transmissions can occur simultaneously.
xDSL gains these advantages over current technology in a relatively simple manner. As mentioned earlier, xDSL utilizes a higher frequency band than POTS. This higher frequency band of 20 kHz to 1.1 MHz is divided into two sections, that is, one for upstream data and one for downstream data. Thus, xDSL is able to allow data transfer in both directions at the same time. FIG. 1 depicts the manner in which the frequency spectrum can be divided for POTS and xDSL. The upstream data spectrum 12 ranges from 20 kHz to 160 kHz, for example, and the downstream data spectrum 10 ranges from 240 kHz to 1.1 MHz. In accordance with xDSL, the upstream and downstream spectrums 12 and 10 are further divided into 256 4.3 kHz blocks. These blocks are referred to as xe2x80x9ctonesxe2x80x9d. The downstream spectrum 10 contains more tones and thus has the capability of transmitting data faster. The reason for further dividing the spectrums into tones is so that, if interference noise exists at a certain frequency and the data associated with the tone at that frequency is being destroyed, an xDSL system can refuse to transmit data on that tone. The system will then use a different tone to transmit the data safely. If this should occur, the xDSL system does not transmit data at its maximum rate; however, data integrity is high. When an xDSL system is first powered on, the system checks all of the tones available in the frequency spectrums 10 and 12 to see if data can be transferred on each tone. If the system finds that a sufficient number of good tones are available, the system is said to be xe2x80x9ctrainedxe2x80x9d, and data can be transmitted.
With this new DSL technology, new problems have also arisen. A problem with xDSL transmissions which is currently foreseeable in virtually all residential and commercial facilities is noise. Essentially all of these facilities have electronic devices (e.g., motor driven devices and variable switches) which generate noise in the form of electromagnetic interference (EMI). Additional examples of these electronic devices include TRIAC devices found in light dimmers and hair-dryers, and brushes in electric motors located in ceiling fans and air-compressors in refrigerators, heat-pumps, and so on. This noise is generated in a frequency range of approximately 10 kHz to 5 MHz, encompassing the entire xDSL spectrum. This poses a problem with existing unshielded twisted pair copper telephone lines because the household EMI noise couples onto these telephone lines and can potentially destroy any xDSL transmission. If the EMI noise is very severe, other network xDSL subscribers may also be degraded from crosstalk in the network feeder cable""s binder group.
A need therefore exists for a method that relocates the xDSL sign spectrum away from the most severe EMI noise spectrums located on the customer premises. Shifting the frequency range of the xDSL transmission above the EMI noise range, however, also poses problems. Existing unshielded twisted pair telephone lines have a frequency response similar to a low-pass filter. If the xDSL frequency range were shifted up above the EMI noise range, much of the transmission could be lost due to the low-pass filter effects of the twisted pair telephone lines. The problem also increases as the length of the telephone line increases. As the length of the telephone line increases, the cutoff frequency of the telephone line decreases, which increases the amount of data that could be lost. A need exists for an xDSL system which addresses and solves this problem, as well.
In addition, when an xDSL set-top service is supplied for Video On Demand (VOD) within a customer""s premises, the connection is specified to be UTP or STP. The CPE connection from the network interface device (NID) to the television (TV) or set-top box, however, is typically coaxial. A method is needed to reuse the existing customer premises coaxial wiring to deliver the xDSL service from the NID to the xDSL set-top box.
Also, UTP wiring within the customer premises is an uncontrolled element to the network provider. This uncontrolled element has the potential to radiate an xDSL signal much like an antenna radiates a signal, thereby violating FCC emission limits. A method is needed to isolate the existing POTS wiring from the network without having to install a dedicated drop for the xDSL service.
In order to keep EMI noise from potentially destroying an xDSL transmission, an xDSL spectrum relocation system is provided in accordance with the present invention which shifts the frequency range of an xDSL transmission above the EMI noise range. Also, to solve the problem of the low-pass filter characteristics of the existing twisted pair telephone lines, the system of the present invention utilizes the existing coaxial cable TV lines within the house.
The xDSL spectrum relocation system of the present invention shifts signals from one form of wire to another. With the implementation of xDSL, the incoming telephone line to a house carries both POTS and xDSL transmissions. The xDSL signal is removed from an upstream telephone line and then shifted up in frequency using amplitude modulation (AM) to approximately 25 MHz, which is well above the household EMI noise range. Amplitude modulation essentially uses the amplitude of the data signal to vary the amplitude of a carrier signal. Thus, the data and carrier signals appear to be one signal being transmitted at the frequency of the carrier. This signal is then placed on coaxial cable at a NID with the existing cable TV signals. Since existing cable TV frequencies begin at about 50 MHz, there is no interference between the two signals.
Because the existing standard for xDSL does not involve signals outside a selected frequency range (e.g., signals outside a range between 20 kHz and 1.1 MHz), the signal from the coaxial cable is demodulated after it has been transmitted through the house without being affected by EMI noise. Thus, the signal is removed from the coaxial cable, demodulated and placed back on twisted pair telephone line so that the existing xDSL modems can accept the signal.
The system of the present invention comprises two devices located at the customer premises which provide spectrum relocation of xDSL signals. Other cable and active xDSL devices do not require modification. The first device is a spectrum relocating NID which can be substituted for an existing POTS ND and the existing CATV NID. This four-port device provides two network side connections and two customer premises connections. The four-port NID device supports a number of features and functions such as: a spectrally band-limited (i.e., for POTS and xDSL) UTP port for connection to the public network; a spectrally band-limited (Le., for broadcast CATV and upstream pay-per-view PPV signaling coaxial port for connection to the public CATV provider network; a spectrally band-limited (i.e., for POTS only) MPT port for connection to existing customer premises POTS wiring; a spectrally band-limited (i.e., for CATV, PPV signaling, and relocated xDSL) coaxial port for connection to existing customer premises coaxial wiring, a function to relocate the downstream xDSL signal (i.e., a network-to-CPE signal) from its normal spectral location on the network side UTP port to an unused spectral location between 5 and 30 MHz on the customer premises side coaxial port; a function to relocate the upstream xDSL signal (Le., a CPE-to-network signal from an unused spectral location between 5 and 30 MHz on the customer premises side coaxial port to its normal xDSL spectral location on the network side UTP port; a function to pass through the POTS signal between the UTP ports on the network and customer premises sides; and a function to pass through the CATV signals (i.e., downstream and PPV upstream signals) between the coaxial ports on the network and customer premises sides. All conventional NID functions such as grounding and surge protection provided by existing types of POTS and CATV NIDs are provided by the new four-port NID.
The second spectrum relocating device of the present invention is a CPE interface device co-located with an existing CPE xDSL device such as a personal computer (PC) network interface card (NIC), an xDSL modem, or an xDSL VOD set-top box. The CPE interface device supports a number of features and functions such as: a spectrally band-limited (i.e., for xDSL only) UTP port for connection to the CPE xDSL termination device; a spectrally band-limited (i.e., for CATV, PPV upstream, and relocated xDSL signaling) coaxial port for connection to customer premises existing coaxial wiring; a spectrally band-limited (Le., for broadcast CATV and PPV upstream signaling) coaxial port for connection to a co-located TV or existing set-top device; a function to relocate the downstream (network-to-CPE) xDSL signal from its relocated spectral position between 5 and 30 MHz on the customer premises coaxial port to its normal spectral position on the UTP port connecting to the xDSL CPE device; a function to relocate the upstream xDSL signal (i.e., a CPE-to-network signal from its normal xDSL spectral position on the UTP port connecting to the xDSL CPE device to an unused spectral location between 5 and 30 MHz on the customer premises coaxial port; a function to provide a carrier signal between 5 and 30 MHz over the customer premises coaxial wiring to be used by the NID to obtain power and local oscillator function for frequency relocation; a function to pass through the CATV signals (i.e., downstream and PPV upstream signals) between the coaxial ports for the customer premises coaxial wiring and the co-located TV or set-top device; and a standard UL compliant AC (117 volts and 60 Hz) power connection.
By shifting of the xDSL spectrum to a lower noise frequency band on the in-home coaxial wiring, the above-mentioned problem areas are minimized. Also, by keeping the NID device simple and mostly passive, high reliability and low cost can be achieved, thereby meeting the traditional design goals of NIDs owned by the Public Switched Network (PSN).